Fuel cell and membrane

ABSTRACT

Fuel cells and a novel membrane for use in fuel cells and manufacturing processes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application,Ser. No. 10/017,140 filed Oct. 30, 2001 which claims the benefit of thefiling of U.S. International Application, Ser. No. PCT/US00/12510 filedMay 5, 2000, entitled Fuel Cell and Membrane, which further claims thebenefit of U.S. Provisional Patent Application Ser. No. 60/132,909,entitled Micro Fuel Cell Based on Planar Interdigitated Electrodes,filed on May 6, 1999, and the specifications thereof are incorporatedherein by reference.

GOVERNMENT RIGHTS

The Government has rights to this invention pursuant to Contract No.DE-AC04-94AL85000 awarded by the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

Field of the Invention (Technical Field)

The present invention relates to fuel cells and membranes.

SUMMARY OF THE INVENTION (DISCLOSURE OF THE INVENTION)

The present invention comprises novel fuel cells, porous material foruse therein and methods of making the same. The present invention alsocomprises novel methods for embossing and molding materials. The presentinvention further comprises a combinatorial procedure for expeditiouslytesting designs.

In a preferred embodiment, a fuel cell, which may be formed entirely bysemiconductor manufacturing methods, comprises a dielectric substratematerial with a porous film disposed on its upper surface, wherein thefilm comprises at least one electrode, and channels extend through thedielectric material. A fuel source is disposed in relation to thechannel aperture, preferably on a lower surface of the dielectricmaterial. The fuel source of the preferred embodiment comprises hydrogengas, alcohols, O₂, or other compounds containing redox pairs, includingambient air.

The porous film of the present invention comprises pores betweenapproximately 5 nm and approximately 1000 nm in diameter, but preferablyat least 0.18 μm in diameter. The porous film of the preferred fuel cellembodiment is prepared through etch-processing and comprises a solidelectrolyte. The solid electrolyte may be disposed on the dielectricmaterial by spin coating, lamination, or spraying. The film may be aproton exchange polymer such as Nafion® which may be used in conjunctionwith a moisture cap. However, the solid electrolyte may be a hightemperature proton conducting electrolyte or an oxide conductingelectrolyte. Oxide conducting electrolytes may comprise zirconia-basedelectrolytes and have high operating temperatures between approximately100° C. and approximately 1000° C.

The porous film of the preferred embodiment may comprise a silicon-basedthin film membrane, such as silicon nitride or silicon carbide. Themembrane is a low stress, pre-tensioned membrane. The thin film membraneis between approximately 0.5 μm and approximately 20 μm in thickness andcomprises a patterned and etched geometric array having an approximately1 μm diameter and an approximately 2 μm pitch. The thin film is formedthrough low pressure chemical vapor deposition having pores formedthrough reactive ion etching. Such a thin film may be used as a mask foranodization etching processes.

The porous film may comprise a conductive layer utilizing gold,aluminum, platinum, other metals, metal alloys, or conductive organicmaterials. The porous film may further comprise a catalyst disposed onthe solid electrolyte which may be a refractory material, such asplatinum.

This catalyst disposition on the porous film results in a porouscatalyst. A high temperature catalyst, e.g., Noble metals, non-Noblemetals, metal oxides, and oxide compositions may be used. Noble metalsof the preferred embodiment are selected from the group of Pt, Au, Ag,Pd, and Ag/Pd alloys. Non-Noble metals of the preferred embodiment areselected from the group of Ni, Co. Cu, and Fe. Metal oxides of thepreferred embodiment are selected from the group of PrO₂, CeO₂, andInO₃. Preferred oxide compositions are manganites and cobaltites. Theconductive layer and/or catalyst are disposed by chemical vapordeposition, physical deposition, evaporation, and ink deposition.

The at least one electrode of the present invention preferably comprisesat least one anode and at least one cathode, wherein the anode andcathode may have different surface areas. In the preferred embodimentthe anode surface area is between approximately two times andapproximately ten times less than the cathode surface area, morepreferably, it is approximately four times less than the surface area ofthe cathode. In the most preferable embodiment, the anode isapproximately 40 μm wide by approximately 1 cm long, and the cathode isapproximately 160 μm wide by approximately 1 cm long.

The dielectric substrate may be a silicon-based material, preferablysilicon nitride. The channels may be pores, preferably formed byreactive ion etching, within the dielectric substrate which aresurrounded by the substrate material serving as a dielectric barrier. Inthe preferred embodiment, every other dielectric barrier between ananode and a cathode additionally comprises a conductive layer coating.The dielectric barrier may comprise a width of between approximately 10μm and 50 μm. In the most preferred embodiment, the width isapproximately 25 μm. The resultant pores comprise at least one flow pathfor fuel to reach electrodes. Preferably, the aperture of thechannels/pores corresponds to the surface area of the electrode. Thechannels of the present invention may be formed by joining at least twomicromachined wafers.

The fuel cell of the present invention comprises geometric surfacesselected from the group of planes, curved surfaces, flexible surfaces,and cylinders. The apertures of the cylinders may comprise geometricfigures comprising triangles, rectangles, circles, polygons, andellipses.

The fuel cell of the present invention may comprise a bipolar cellcomprised of two fuel cell units each comprising a dielectric substrate,a porous film disposed thereon, wherein the porous film comprises atleast one electrode. In this bipolar cell embodiment, one unit'selectrode comprises a cathode and the other comprises an anode.

Fuel cells of the present invention may have anodes and cathodesinterposed in interdigitated planar array serpentine, or spiralpatterns, and/or comprise parallel, series, or parallel-seriesconfigurations.

In the present invention, the fuel cell may additionally comprise atleast one ohmic contact. The contract may comprise a coating on thelower surface of the dielectric substrate material. The coating may beselected from the group of aluminum, gold, silver, other metals, andmetal alloys.

The fuel cell of the present invention may additionally comprisemicroswitching devices wherein, preferably, the devices selectivelyinterconnect electrodes. These devices may be disposed within thechannels to control fuel flow.

The preferred fuel cell comprises an etch and anodization processedporous electrode which may have cooling means to reduce fuel celltemperature and may be formed by semiconductor manufacturing methods.When the electrode is silicon-based, the silicon may be doped.

The present invention additionally comprises methods for making a fuelcell. In a preferred embodiment, forming a silicon-based electrodecomprises the steps of providing a silicon-based substrate, etching atleast one side of the silicon-based substrate, and anodizing thesubstrate and embossing a substrate comprising the steps of providing asupport substrate forming a film on the support substrate, patterningfeatures of the film, providing a second substrate, and embossingfeatures of the film into the second substrate. The patterning maycomprise adding and/or subtracting material from the film.

In a preferred embodiment of the present invention, making a moldincludes the process of forming a fuel cell, including the steps ofproviding a silicon substrate, patterning the silicon substrate, andcontacting the silicon substrate with a deformable material therebyimparting the pattern to the deformable material.

Further, the present invention comprises a lithography processing methodof processing a substrate wafer which utilizes front and back sideprocessing. Front side processing includes the steps of providing aclean wafer, depositing a film on the wafer depositing a photoresistlayer onto the film; masking the photoresist layer with a geometricarray, exposing the masked photoresist layer, developing the photoresistlayer, rinsing the photoresist layer, and etching the layered wafer. Thefilm may be disposed through chemical vapor deposition, physicaldeposition, or sputtering, and may be silicon-based, preferably siliconcarbide. Further, etching is preferably accomplished through reactiveion etching. Back side processing includes the steps of providing aclean wafer, depositing a photoresist layer, waiting at leastapproximately an hour, masking the photoresist layer with a geometricarray, aligning the mask with the masking of the front side, exposingthe photoresist layer, developing the photoresist layer, rinsing thephotoresist layer, and etching the layered wafer. The film may bedisposed by sputtering and baking the wafer. As with the front sideprocessing, the etching process in back side processing may be done byreactive ion etching which may comprise alternate polymer deposition.

Finally, the present invention comprises a packaging method which may beused in processing wherein a small amount of gold is evaporated,deposited onto a ceramic package, and an electrode is attached to thepackage with a gold conductive epoxy.

The fuel cell of the present invention may comprise at least oneetch-processed, conductive, porous film comprising at least oneelectrode wherein the film optionally comprises at least one layer. Theat least one layer optionally comprises at least one dielectric layerand at least one conductive layer wherein the dielectric layeroptionally comprises silicon and the conductive layer optionallycomprises at least one material selected from the group consisting ofgold, aluminum, platinum, and a conductive organic material.

In a preferred embodiment, a fuel cell of the present inventioncomprises at least one catalyst. In a preferred embodiment, theinventive fuel cell comprises at least one support substrate forsupporting the film, and preferably wherein the support substratecomprises at least one fuel flow path.

The present invention also comprises a process for making anetch-processed, porous film comprising the steps of: providing a supportsubstrate; forming a film on the support substrate; and etching pores inthe film; and preferably further comprises the step of etching thesupport substrate; and preferably wherein the film comprises at leastone layer selected from the group consisting of at least one conductinglayer and at least one dielectric layer.

The present invention further comprises an embodiment comprising a fuelcell that comprises an etch and anodization processed, silicon-based,porous electrode, and preferably further comprising at least onecatalyst.

The present invention comprises a method of making a silicon-basedporous electrode comprising the steps of: providing a silicon-basedsubstrate; etching at least one side of the silicon-based substrate;anodizing the etched silicon-based substrate thereby forming pores inthe silicon-based substrate.

The present invention comprises a method of embossing a substratecomprising the steps of: providing a support substrate; forming a filmon the support substrate; patterning features to the film; providing asecond substrate; and embossing features of the film into the secondsubstrate; and preferably wherein patterning comprises at least one stepselected from the group consisting of adding material to the film andsubtracting material from the film.

The present invention comprises a method of making a mold comprising thesteps of: providing a silicon substrate; patterning the siliconsubstrate; and contacting the silicon substrate with a deformablematerial thereby imparting the pattern to the deformable material, andpreferably wherein patterning comprises at least one step selected fromthe group consisting of adding material to the film and subtractingmaterial from the film.

The present invention further comprises a method of combinatorialexperimentation comprising the steps of: providing materials for makingfuel cells wherein the materials comprise a silicon substrate; making aplurality of fuel cells on the silicon substrate; and testing the fuelcells.

In a preferred embodiment, fuel cells of the present invention arecapable of operation at elevated temperatures, for example, temperaturesgreater than approximately 100 C. In a preferred embodiment of a fuelcell of the present invention, anode and cathode electrodes comprisedifferent surface areas that optionally compensate for fuelcharacteristics.

A primary object of the present invention is to satisfy the need forcompact and durable fuel cells.

A primary advantage of the present invention is compactness anddurability when compared to traditional fuel cells.

Other objects, advantages and novel features, and further scope ofapplicability of the present invention will be set forth in part in thedetailed description to follow, taken in conjunction with theaccompanying drawings, and in part will become apparent to those skilledin the art upon examination of the following, or may be learned bypractice of the invention. The objects and advantages of the inventionmay be realized and attained by means of the instrumentalities andcombinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate several embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating a preferred embodiment of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1 a is a diagram of a fuel cell for energy generation;

FIG. 1 b is a diagram of a electrolysis cell;

FIG. 2 a is a diagram of a proton conductor comprising a catalyst;

FIG. 2 b is a diagram of a particle supporting a catalyst;

FIG. 3 is a plot of cell voltage versus current density;

FIG. 4 is a diagram of a preferred embodiment of a planar fuel cell ofthe present invention;

FIG. 5 is a cross-sectional diagram of a preferred embodiment of aplanar fuel cell of the present invention;

FIG. 6 a is a diagram of a preferred embodiment of an electrode array ofthe present invention;

FIG. 6 b is a diagram of a preferred embodiment of an electrode array ofthe present invention;

FIG. 6 c is a diagram of a preferred embodiment of an electrode array ofthe present invention;

FIG. 7 is a diagram of a preferred embodiment of a bipolar cell of thepresent invention;

FIG. 8 is a cross-sectional diagram of a preferred embodiment of a fuelcell of the present invention;

FIG. 9 is a sectional plan view diagram of a flow path of a preferredembodiment of the present invention;

FIG. 10 is a plan view diagram of a fuel cell of a preferred embodimentof the present invention;

FIG. 11 is a plan view diagram of a plurality of fuel cells according toa preferred embodiment of the present invention;

FIG. 12 a is a schematic diagram of a lithography process according to apreferred embodiment of the present invention;

FIG. 12 b is a schematic diagram of a lithography process according to apreferred embodiment of the present invention;

FIG. 13 is an electron micrograph plan view of an etched silicon waferaccording to a preferred embodiment of the present invention;

FIG. 14 a is an electron micrograph side view of an etched silicon waferaccording to a preferred embodiment of the present invention;

FIG. 14 b is an electron micrograph side view of an etched silicon waferaccording to a preferred embodiment of the present invention;

FIG. 15 is a plan view of a portion of an electrode array according to apreferred embodiment of the present invention;

FIG. 16 is an electron micrograph plan view of an anodized siliconmembrane according to a preferred embodiment of the present invention;and

FIG. 17 is a plot of polarization for a fuel cell according to apreferred embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (BEST MODES FOR CARRYING OUTTHE INVENTION)

The present invention comprises fuel cells and a novel membrane for usein fuel cells. A particular preferred embodiment comprises a novelporous thin film membrane whereas an alternative embodiment comprises aporous layer formed through etching of a wafer. Details of thisalternative “wafer membrane” embodiment and the “thin layer membrane”embodiment are disclosed below.

Fuel cells of the present invention are configurable in a variety ofconfigurations. Such configurations include bipolar and planarconfigurations. In general, bipolar configurations comprise a protontransfer material disposed between an anode and a cathode whereas planarconfigurations comprise a proton transfer layer covering a layercomprising at least one anode and at least one cathode. Bipolarconfigurations inherently comprise a sandwich whereas planarconfigurations optionally comprise a sandwich. Bipolar configurationsinherently comprise at least two wafers whereas planar configurationscomprise at least one wafer. For example, in a preferred embodiment of aplanar configuration fuel cell, a single wafer comprises flow paths forhydrogen and oxygen fuels. In an alternative planar configurationembodiment, flow paths for hydrogen and oxygen, or other fuels, areconstructed through use of at least two wafers.

In a preferred embodiment, planar configuration fuel cells comprise aporous thin layer membrane. This thin membrane comprises, for example,pores created from use of an etching process. This membrane furthercomprises gold and platinum wherein gold serves as an electronconductive material and platinum serves as a catalyst. In such apreferred embodiment, a mask is used to selectively apply gold to areasof the thin membrane and another mask is used to selectively applyplatinum to certain gold covered areas of the thin membrane. The thinmembrane selectively comprises at least one anode and at least onecathode. Of course for a bipolar fuel cell, the thin membrane optionallycomprises at least one anode and/or at least one cathode.

In a preferred embodiment, fuel cells of the present invention compriseat least one porous silicon membrane. According to the presentinvention, a porous silicon membrane is fabricated through an etchingprocess wherein a first side of a, for example, silicon wafer on which asilicon carbide thin film has been deposited is etched to exposeconductive silicon and a second side is etched using a processcomprising, for example, but not limited to, Bosch reactive ion etching.The first side comprises etching to a depth of approximately 1 micronwhereas the second side comprises etching to a depth to withinapproximately 50 microns of the first side. The exposed silicon is madeporous by a porous silicon anodization process. The resultingapproximately 50 micron porous silicon region disposed between the firstand second sides comprises a porous membrane. This membrane is coatablewith, for example, a catalyst. In a preferred embodiment, a fuel cell isformed by flowing fuels through two platinum coated porous wafermembranes wherein a proton conductive layer is sandwiched there between.

A bipolar device 40 suitable for use as a fuel cell, as shown in FIGS. 1a and 1 b, comprises an electrolyte layer 46 sandwiched between an anode42 and a cathode 44. The device 40 is generally operable in two modes: afuel cell mode and an electrolysis mode. In the fuel cell mode, hydrogenand oxygen are consumed thereby producing water and energy. In the fuelcell mode, energy is available as an electrical potential across anode42 and cathode 44. In the electrolysis mode, water and energy areconsumed thereby producing oxygen and hydrogen. In the electrolysismode, energy is applied across the anode 42 and the cathode 44. Ineither mode, diffusion and/or transport of constituents are designconsiderations. The present invention is not limited however to hydrogengas and oxygen gas fuels; methanol and other fuels (supplied as solid,gas, and/or liquid) are also within the scope of the present invention.

A schematic showing diffusion of water, hydrogen and oxygen is shown inFIG. 2 a and FIG. 2 b. FIG. 2 a shows a cross section of an electrode 60while FIG. 2 b shows diffusion around a single particle 68. As shown inFIG. 2 a, a layer of carbon supported 62 platinum catalyst 64 isdisposed on the surface of a membrane 66. In this particular example,the membrane 66 comprises a NAFION® material (E. I. Du Pont de Nemours,Del.—a porous plastic material) that allows for transport of hydrogenatoms (protons) through an aqueous phase 70 comprising, for example,water. Of course, the present invention is not limited to the use ofNAFION® material; thus, other materials that can transport hydrogenions, especially those which are non acqueous solid electrolytes andwhich can tolerate temperatures above 100 C., are within the scope ofthe present invention. Other materials comprise, for example, but arenot limited to, silicon-based material comprising organic materialcomprising proton donors are within the scope of the present inventionas alternatives to NAFION® material. Additionally, while platinum is thepreferred catalyst, other materials such as but not limited to Ru or aPt—Ru alloy may be utilized. FIG. 2 b shows the transport of protons andoxygen through the aqueous phase 70 to the catalyst 64 where water isformed.

FIG. 3 shows a plot of fuel cell voltage versus current density. For a“perfect” fuel cell, cell voltage is constant over current density anddescribed by the free energy divided by the product of number ofelectrons transferred and Faraday's constant. In reality, cell voltageis not constant over a range of current density. Instead, cell voltagedecreases from a theoretical value (including, for example, competingreaction losses) to a minimal value that approaches zero as currentdensity is increased. At first, cell voltage decreases due to activationlosses due to kinetic concerns. Next, at a higher current density, Ohmiclosses dominate thereby leading to a further decrease in cell voltage.Thereafter, at higher current densities, diffusion or concentrationlosses dominate. According to the present invention, a novel fuel cellcomprising novel materials overcomes limitations that, in part, lead todecreased cell voltage.

A preferred embodiment of a planar fuel cell is shown in FIG. 4. In thisembodiment, the planar fuel cell 80 comprises an anode and cathode array82 that is in contact with a proton conducting membrane 84. In thisembodiment, the array 82 is further in contact with a source of hydride86 and the membrane 84 is in contact with a cap 88. In a preferredembodiment, the cell comprises a hydride source 86 upon which an array82 is placed, upon which a proton conductor 84 is placed, upon which acap 88 is placed. An inventive process for manufacturing fuel cells ofthe present invention allows for manufacture of such cells using thematerials, processes and equipment common to the semiconductor industryand in multi-chip modules or in sheets comprising a plurality of fuelcell modules 90. The planar fuel cells shown in FIG. 4 are optionallymodifiable through, for example, elimination of the cap 88 and placementof an opposing array 82 onto the membrane layer 84 thereby forming asandwich configuration. A bipolar configuration comprising one arrayserving as an anode and a second opposing array serving as a cathode isalso within the scope of the present invention.

In a preferred embodiment, the fuel cell comprises a planar structure.Such an embodiment of a fuel cell 100 is shown in FIG. 5. As shown inFIG. 5, this cell comprises a silicon cap 102. This cap 102 comprisesfuel flow paths, for example, flow channels 104. In this particularembodiment, the cap 102 comprises at least two independent paths 104,for example, one for hydrogen and another for oxygen or air. A porousmaterial 106 covers the flow paths 104. Adjacent to the porous materialor deposited thereon is at least one catalyst layer. In this embodiment,the catalyst layer forms a catalyst electrode 108. In a preferredembodiment, the at least one catalyst electrode 108 comprises platinum,however, other catalysts, including but not limited to Ru or a Pt—Rualloy may also be used. Fuel diffuses and/or is transported through theporous material 106 and reaches the at least one platinum anodeelectrode 108 (e.g., hydrogen fuel) and at least one platinum cathodeelectrode 108 (e.g., oxygen fuel) whereby, for example, water and anelectrical potential between the anode and cathode electrodes aregenerated. Also shown in FIG. 5 are at least one electrolyte material110, 112 and a cooling fluid 114. One of these layers, or alternativelyan additionally layer, comprising the ability to control membranemoisture is within the scope of the present invention. Moisture controlis germane when a NAFION® membrane is used because this class ofmembrane materials rely on moisture to facilitate proton transfer. Inalternative embodiments of the present invention, proton transfermembranes that do not rely on moisture are used. Such membranestypically operate at temperatures greater than the operatingtemperatures of the NAFION® class of membranes.

Various embodiments of electrode configurations of the present inventionare shown in FIG. 6 a, FIG. 6 b and FIG. 6 c. As shown in FIG. 6 a, theelectrode array 120 comprises a porous film cathode 122 and a porousfilm anode 124. In a preferred embodiment, these films 122, 124 compriseplatinum, but may be comprised of other catalytic materials. The widthof each of the four legs of each electrode is, in a preferredembodiment, on the order of approximately 5 microns. The invention isnot limited to anode and cathode electrodes of equal size and/ordimensions. For example, anode area optionally comprises an area lessthan that of cathode area. In a preferred embodiment, anode areaoptionally comprises from approximately two times less to approximatelyten times less the cathode area. Depending on the type of fuel, anembodiment comprising an anode area greater than a cathode area iswithin the scope of the present invention. As shown in FIG. 6 b and FIG.6 c, electrodes are optionally oriented in parallel, in series, oralternatively, a combination of both.

A bilayer or bipolar structure of a silicon-based porous electrode ofthe present invention 140 is shown in FIG. 7. According to thisembodiment, preferably a platinum catalyst 142 is deposited on thesurface of a silicon wafer comprising a porous membrane 144. A protonconducting material 146, such as NAFION®, is placeable in contact withthe platinum catalyst layer 142. Further, a silicon wafer of optionallyidentical structure 148 is placeable in contact with the porous material146. In this embodiment, one catalyst coated porous wafer serves as ananode and the other catalyst coated porous wafer serves as a cathode.Hydrogen gas is made to flow through the anode gas diffusion electrodefrom the back. The cathode is air breathing, and allows waste water toexit the electrode through the back.

A cross section of a planar silicon fuel cell of a preferred embodimentof the present invention is shown in FIG. 8. This cell 160 comprises twosilicon wafers 166, 168 comprising flow paths for hydrogen 162 and flowpaths for air 164. The two wafers 166, 168 are manufactured, forexample, micro-machined, and then bonded together to form the flow paths162 and 164 (shown in an exploded view in FIG. 8). The horizontal arrowsrepresent flow through a bend, for example, the bends comprising aserpentine path as shown in FIG. 9 and FIG. 10. A layer comprisingetched pores 170, catalyst and current collector 174, is integral withthe upper silicon wafer 168 and in contact with a proton exchangematerial 172, comprising for example a proton exchange polymer. In apreferred embodiment, the current collector comprises gold and thecatalyst comprises platinum deposited on porous gold covered electrodes.However, other metals or metal alloys (e.g., aluminum, copper, steel,etc.) may comprise the current collector and other catalytic materialsmay be utilized.

A plan detailed view of the back side of the top wafer of a planarsilicon fuel cell of a preferred embodiment of the present invention isshown in FIG. 9. This embodiment comprises an inlet for hydrogen 180 andoptionally an outlet for hydrogen, not shown. Hydrogen gas entering thecell contacts at least one anode 182 whereas at least one air breathingcathode 184 is interposed between the at least one anode 182. The atleast one cathode 184 is open to the atmosphere allowing air to enter,and comprises permeability to water. In a preferred embodiment, the atleast one anode and the at least one cathode comprise individual pathwidths on the order of approximately 40 microns wherein a barrierseparates the cathode and anode path by a width on the order ofapproximately 25 microns. In a preferred embodiment, the barriercomprises a dielectric barrier. Of course, anode and cathode widths areoptionally variable by approximately an order of magnitude greater orlessor wherein anode and cathode widths are not necessarily equal.Furthermore, width may vary as a function of length. A similar butexpanded view of a cell as shown in FIG. 9 is shown in FIG. 10. Thepresent invention is not limited to such serpentine configurations;spiral and other configurations are within the scope of the presentinvention. FIG. 11 shows 18 cells on a single wafer. The presentinvention allows for manufacture of cells comprising differentproperties; therefore, all 18 cells are optionally the same ordifferent, including grouped different and individually different.

Silicon Structures

Silicon structures of the present invention are manufactured usingstandard silicon fabrication techniques, including, but not limited to,lithography. Referring to FIG. 8, a preferred embodiment of the presentinvention comprises silicon structures comprising fuel flow paths and alayer comprising etched pores, for example, a thin layer membrane. Inthis particular embodiment, the layer comprising etched pores comprises,for example, a thin layer of silicon nitride that has been depositedonto a wafer. In an alternative embodiment, a porous layer is formed bypatterning and reactive ion etching pores in a silicon nitride thinfilm, and patterning and etching the silicon wafer from the back using aprocess comprising for example, Bosch reactive ion etching. In thisalternative embodiment, the front side and back side of a wafer areetched to create a porous membrane. Details of this alternative “wafermembrane” embodiment are disclosed below followed by details of the“thin layer membrane” embodiment. Of course, some of the processes andfeatures of one embodiment are applicable to the other.

Wafer Membrane Embodiment

In a preferred embodiment comprising lithography, the front of a siliconwafer is processed differently than the back of a silicon wafer. Suchprocesses are depicted in FIG. 12 a and FIG. 12 b for front and backprocessing, respectively.

Front Side Processing

The process depicted in FIG. 12 a comprises the steps of providing aclean wafer; depositing through chemical vapor deposition (“CVD”), orother suitable process, an approximately 0.5 micron thick film onto thewafer, for example, a silicon carbide film or other film that cansurvive anodization in hydrogen fluoride; depositing a photoresist(“PR”) layer onto the film; masking the PR layer with an array of, forexample, approximately 1 cm circles; exposing the masked PR layer forapproximately 6.5 seconds; developing the PR layer; rinsing thedeveloped PR layer; and etching the layered wafer using reactive ionetching (“RIE”).

Back Side Processing

The process depicted in FIG. 12 b comprises the steps of: providing aclean wafer; depositing a PR layer, comprising, for example, spinning atapproximately 3,500 rpm for approximately 30 seconds and baking forapproximately 3.5 minutes at approximately 110° C.; waiting at leastapproximately one hour before performing a next step; masking with amask comprising an array of circles and/or polygons, for example,hexagons, wherein masking further comprises aligning the mask to indiciaon the front side of the wafer; exposing the masked PR layer forapproximately 10 seconds; developing the PR layer in, for example, anapproximately 4:1 AZ type developer for approximately 2.5 minutes;rinsing the PR layer; and etching the layered wafer using Bosch reactiveion etching to achieve, for example, a thickness of betweenapproximately 50 microns and approximately 100 microns between the frontside and the bottom of the holes created by the process. The last step,Bosch reactive ion etching, optionally comprises alternate polymerdeposition and etching wherein, for example, the end effect is to removepolymer from horizontal surfaces but not from vertical surfaces (e.g.,side wall surfaces). FIG. 13 shows an electron micrograph plan view of ahexagonally etched wafer. FIG. 14 a and FIG. 14 b show electronmicrograph side views of a hexagonally etched wafer.

Anodization

After front and back side processing, the wafer is anodized. In apreferred embodiment, anodizing comprises anodizing at approximately 80mA per square centimeter in an approximately 1:1 solution ofapproximately 49% by weight of hydrogen fluoride and ethanol for a timesufficient to allow pores to reach the bottom of the etched back sideholes. Pores typically grow at approximately 2.6 microns per minute in0.01 Ohm-cm n-type wafers.

Front Side Catalyst Deposition

In a preferred embodiment, platinum is deposited through chemicaldeposition of platinum from platinum salt solutions onto the front sideof a wafer. In an alternative embodiment, a sputtering process is usedto produce, for example, a layer of platinum or other catalyst ofapproximately 100 to approximately 300 Ångstroms onto porous silicon.

Back Side Ohmic Contacts

According to a preferred embodiment of the present invention, Ohmiccontact is formed on the back side of a wafer. For example, an Ohmiccontact comprising a layer of approximately 0.5 microns of a conductivematerial such as aluminum, gold, silver, copper, other metals, or metalalloys is achievable through a sputtering process.

Packaging

In a preferred embodiment, gold conductive epoxy is used to attach asilicon electrode to a gold coated ceramic package. Gold coating of theceramic package is achievable, for example, through an evaporationprocess wherein approximately 0.5 microns of gold is evaporated anddeposited onto a ceramic package.

Membrane Fabrication

In a preferred embodiment of the present invention, a membrane solution,such as, but not limited to a solution of NAFION®, is coated onto eachelectrode at, for example, a temperature of approximately 70° C. Thesolution is spun on, or otherwise coated onto, the electrodes until anapproximately 10 to 50 micron thick membrane layer is formed. While themembrane solution is still sticky, two electrode are pressed together toform a fuel cell, such as a bipolar fuel cell.

Preferred embodiments of fuel cell components shown in the accompanyingfigures comprise a honeycomb support structure, similar to that of abeehive, for added integrity. Support structures comprising shapes otherthan honeycomb are within the scope of the present invention.

Thin Layer Membrane Embodiment

Fuel cells of the thin layer membrane embodiment of the presentinvention optionally comprise at least one novel low stress thin filmmembrane comprising, for example, silicon nitride. In general, themembrane comprises an etch-processed, porous film that optionallycomprises a conductive material, a dielectric material and/or both inthe form of layers or otherwise. According to preferred embodiments ofthe present invention, such a thin film membrane is supported on itsedges, like a drumhead. Such a film is created through, for example, alow pressure chemical vapor deposition (“LPCVD”) process known in theart. According to the present invention, the thin film is less thanapproximately 20 microns, preferably less than approximately 10 microns,and most preferably less than approximately 5 microns. In a preferredembodiment, the thin film comprises a thickness of approximately 1micron. In a preferred embodiment, the membrane is patterned, forexample, with 1 micron diameter circles on a two micron pitch. Thepattern, or mask, allows for creation of pores of same and/or similarsize. Pores are created through, for example, an etching process, suchas, but not limited to, a process comprising reactive ion etching (RIE).

FIG. 15 shows a section of a cell 220 comprising three completeapproximately 40 micron wide electrodes 222 at approximately 50 timesmagnification. These electrodes comprise an approximately 1 micron thicksilicon nitride thin film that has been etched with pores comprisingapproximately 1 micron diameters on approximately 2 micron centers.Conductive strips 224 to the right of each electrode are goldinterconnects. The area between the electrodes are approximately 25micron wide dielectric barriers 226 that insulate the electrodes fromone another electrically. They also prevent, for example, hydrogen andoxygen, which arrive at the electrode surfaces from the bottom of thewafer, from mixing. Although this preferred embodiment comprisescircles, the present invention is not limited to circle shaped pores.

The embodiment shown in FIG. 15 is also easily modified to create a celllike a bipolar plate of a conventional fuel cell stack. For example,every other barrier between anode and cathode is optionally coated witha conductive layer. Alternative embodiments allow for micro-switchingdevices that can selectively interconnect anodes and cathodes dependingon the desired output. Such output optionally comprises series and/orparallel connections. Micro-switching devices for controlling the flowof fuel are also within the scope of the present invention.

In a preferred embodiment, pores are etched into and through the thinfilm using reactive ion etching (RIE) thereby creating an artificialporous membrane. The pores optionally comprise circles comprisingsmaller and/or larger diameters and/or a greater pitch. Of course,polygonal and ellipsoidal pores are within the scope of the presentinvention. Fuel cell anodes are spanned by such an artificial porousmembrane and are, for example, approximately 40 microns wide andapproximately 1 centimeter long, and cathodes are, for example,approximately 160 microns wide and 1 centimeter long. In this particularexample, the cathodes surface area is approximately four times as largeas the anode surface area. In such an embodiment, the area for oxygeninteraction with catalyst is increased relative to the total area ofanode and cathode combined. In many instances, the slower kinetics ofthe catalytic reaction at the cathode limits the power density of a fuelcell. Thus, an increase in cathode area with respect to anode areapromotes overall power density. Of course, fuel cell plenums that supplyfuel to an anode and/or a cathode are optionally sized to correspond toanode and/or cathode size. Thus, asymmetrical fuel cell plenums arewithin the scope of the present invention, for example, fuel cellplenums are optionally arranged asymmetrically side-by-side in a plane.These plenums provide, for example, access for fuel such as, but notlimited to, hydrogen and oxygen. The plenums optionally comprise equalsizes or different sizes.

The slow kinetics of the ORR is the significant contributor to losses ina PEMFC under current. By increasing the ratio of cathode area to anode,this effect can be at least partially mitigated. This is impossible todo with a bipolar fuel cell design, but in the case of the of the planarfuel cell design, unequal cathode and anode areas becomes possible anddesirable to increase the performance of the fuel cell. Thus, apreferred embodiment of the present invention comprises an approximately160 micron wide oxygen cavity and a hydrogen cavity of approximately 40microns in width, giving an approximately 4:1 ratio of oxygen tohydrogen cavity width. This ratio can be easily varied by mask layout,which is a feature of a planar fuel cell.

The pores in the thin film allow for gas diffusion and/or gas transport.In a preferred embodiment, the porous thin film membrane is coated witha catalyst. For example, hydrogen diffuses through the pores, arrives onthe catalyst and ionizes to two protons. The protons generated at theanode may then enter an ionomeric membrane, which is in intimate contactwith the catalyst containing membrane. Electrons released by hydrogenionization enter a current collector, preferably comprised of gold,thereby allowing them to flow off the anode electrode. In the case ofthe cathode, the membrane is coated with a catalyst which is in intimatecontact with the ionomeric membrane. Oxygen molecules diffuse and/or aretransported through the artificial porous membrane, where they ionize onthe catalyst. Protons which originated on the anode enter the ionomericmembrane thereby causing proton diffusion to the cathode, where protonscombine with the oxygen ions and an electron from the current collector,to form a water molecule, completing the electrochemical process of thefuel cell. The catalyst-facilitated, electrochemical reaction generatesan electrical potential across anode and cathode, which in turn, canproduce current flow. As mentioned above, the present invention is notlimited to a single anode or a single cathode, or a single cell.Multiple anode cells and/or multiple cathode cells, as well as multiplecell fuel cells are within the scope of the present invention.

Embodiments of the present invention that comprise novel porous thinfilm membranes overcome limitations found in conventional state of theart proton exchange membrane fuel cells, mainly because, conventionalmembrane porosity is difficult to engineer. The porous thin filmmembranes of the present invention are engineered according to standardlithography wherein pore size is limited by the lithography process.Modern mass production lithographic techniques known in the art arecapable of producing features at least as small as approximately 0.18micron. Therefore, membranes of the present invention comprising porescomprising, for example, approximately 0.18 micron diameters are withinthe scope of the present invention. Of course, the present invention isnot limited to the current state of the art since it is expected thatnew technology will allow for the making of even smaller features.Basically, as lithographic and other techniques progress, pores can befabricated with even smaller sizes and greater pitches.

As shown in FIG. 15, the thin film comprises gold wherein thedistribution of gold is controlled, for example, through use of a mask.In a preferred embodiment, the at least one anode and at least onecathode comprise gold. Gold covered interstices (space between anddefining pores) facilitate electron transport and conductivity with thegold electron collectors.

As mentioned above, the thin film membrane optionally comprises acatalyst. In an alternative embodiment, catalyst was applied to thefront side of a wafer through chemical deposition and/or sputtering. Ina preferred embodiment comprising a thin film membrane, catalyst isapplied through, for example, a physical process such as, but notlimited to, sputtering and/or evaporation and/or through an ink processwherein the catalyst is written onto the thin film. In a preferredembodiment, a platinum catalyst is applied after application of apreferably gold current collector.

As disclosed above, thin layer membranes are suitable for forming avariety of electrode configurations. This trait stems from silicon waferand masking techniques known in the art and novel applications of thesetechniques to form thin layer membranes.

A preferred embodiment of the present invention comprises a method formaking an inventive film. This method comprises the steps of: providinga support substrate, such as, but not limited to a silicon wafer;forming an etch “stop layer” on the substrate, such as, but not limitedto, a silicon dioxide layer approximately 2 microns in thickness;forming a low stress and slightly tensile film on top of the stop layer,for example, a film comprising silicon nitride comprising a thickness ofapproximately less than 5 microns; patterning the film with a mask;etching the film to create pores, comprising, for example, a dry etchprocess comprising, for example, a gas plasma; cleaning off the residualmask material; patterning current collectors on the film with a mask;depositing a conductive material onto the film comprising, for example,an evaporative deposition process; cleaning off the residual maskmaterial; protecting the porous film with a layer comprising, forexample, photoresistive material; depositing/masking a photoresistivematerial on to the non-film side of the support structure; etching thenon-film side of the support structure comprising, for example, a highaspect ratio etch process comprising high selectivity for the stoplayer; cleaning off the residual mask material; removing the stop layer;and cleaning off protective layer from film.

In alternative embodiments, some of the aforementioned steps areoptional. For example, in a preferred embodiment, the film comprises aconductive material, such as, but not limited to, a conductive polymeror the like (e.g., polyacetylene), gold, aluminum and/or platinum. Insuch embodiments, the film optionally comprises an etch stop; therebyeliminating the need for a separate etch stop layer.

In preferred embodiments, the etch-processed film comprises a conductiveporous film. In a preferred embodiment, the etch-processed, conductive,porous film comprises at least one electrode. Again, the etch-processed,conductive, porous film comprises at least one layer wherein the layercomprises at least one layer comprising a conductive layer and adielectric layer.

Fuel cells according to the present invention comprise fuels such as,but not limited to, hydrogen, air, methanol, hydrocarbons, alcohol, andother materials comprising redox pairs.

Flow paths for fuels and waste products comprise a variety ofgeometries, including channels and/or conduits.

Although the term planar is used herein, fuel cells of the presentinvention are not limited to planar geometry. Fuel cells comprisingcurved surfaces, flexible surfaces, and other geometries (such ascylindrical, polygonal, and the like) are within the scope of thepresent invention.

Fabrication of Other Components Using Technology of the PresentInvention

According to the present invention, processed films are useful forinjection molding and/or embossing processes. For example, the compactdisc industry commonly coats a substrate with metal to create a master.The master is then used to emboss compact discs. In most instances, themaster must have ultraflat properties, as known to those in the art, toproduce compact discs of adequate quality. According to the presentinvention, a processed film serves as a master. In a preferredembodiment, the processed film master comprises resilience therebyallowing it to conform to the shape of the object to be embossed. Inanother preferred embodiment, the processed film serves as a mold, forexample, a mold used in injection molding. In yet another preferredembodiment, a mold comprises an etched silicon wafer alone or inaddition to a processed film.

In a preferred embodiment, the present invention comprises a method ofembossing a substrate comprising the steps of: providing a supportsubstrate; forming a film on the support substrate; patterning featuresto the film; providing a second substrate; and embossing features of thefilm into the second substrate; and preferably wherein patterningcomprises at least one step selected from the group consisting of addingmaterial to the film and subtracting material from the film.

In a preferred embodiment, the present invention comprises a method ofmaking a mold comprising the steps of: providing a silicon substrate;patterning the silicon substrate; and contacting the silicon substratewith a deformable material thereby imparting the pattern to thedeformable material, and preferably wherein patterning comprises atleast one step selected from the group consisting of adding material tothe film and subtracting material from the film.

According to the present invention, technology disclosed herein issuitable for patterning, which, for example, includes etching and otherprocesses capable of adding and/or subtracting material from a filmand/or wafer.

Gas Permeation Studies of Wafer Membrane Embodiments

Gas permeation through inventive porous n⁺-Si membranes wasinvestigated. The investigations comprised testing a series of gasesranging from H₂ to Xe, including a nerve gas stimulant dimethyl methylphosphonate (DMMP). Conductance was found to be constant over a poreinlet pressure range of 0.1 to 10 Torr, establishing molecular flow asthe transport mechanism. An approximate correlation was found betweenconductance and the inverse square root of the gas molecular weight, asexpected for molecular flow. Transport rates compare very well withpreviously investigated n⁺-Si membranes.

Comparison of measured and calculated transport rates indicate amembrane porosity of 64%, well within the range of expected porosity. Ananomalous low transition pressure (200 Torr) was observed for thesemembranes where the flow became viscous. One possible explanation forthis observation is the presence of large cross-sectional parameterdefects. Calculations indicate that these defects could be 16 microncylindrical pores at densities as low as two per membrane. Largediameter defects would significantly reduce the efficiency of side wallmediated processes due to a reduction of the number of gas/side wallcollisions during the transit of a molecule.

The results summarized above are disclosed in detail below. The gaspermeation tests were designed to examine the characteristics ofapproximately 50 micron thick membranes formed in n⁺-Si (density ofapproximately 0.005 to approximately 0.02 Ωcm). Five membranes weregenerated by anodizing residual 50 micron thick films formed by KOHetching. The membranes were anodized at an estimated current density ofapproximately 50 mA/cm² for approximately 82 minutes in an approximately1:1 HF/EtOH electrolyte. Membrane conductance values were determinedfrom measurements of the inlet (P_(i)) and outlet (P_(o)) pressures ofthe pores and computing a conductance based thereon (C=(400l/s)/((P_(i)/P_(o))−1) where 400 l/s corresponds to the pore outletpumping speed, which is equivalent to the system pumping speed). Inletpressures were measured with a temperature stabilized capacitancediaphragm gauge while outlet pressures were measured by a N₂ calibrated(NIST traceable) ion gauge. The ion gauge was calibrated for the othergases investigated by measuring a gauge sensitivity using a N₂calibrated (NIST traceable) spinning rotor gauge.

Results show that gas transport through the membranes is molecular forpressures below 10 Torr. The conductance varies as a function of the gastype and, as expected for molecular flow, is independent of pressure.The subtle oscillations seen in plots of conductance versus pressure arethe result of discrete pressure steps and timing offsets and timeconstants for the gauges used to measure P_(i) and P_(o).

The measured area conductances (normalized to membrane area) of thesemembranes are 5.16×10⁻³ and 1.68×10⁻³ l/cm²s for He and O₂,respectively. These values compare favorably to He and O₂ valuespreviously reported as 5.16×10⁻³ and 1.85×10⁻³ l/cm²s [Memo,“Preliminary Results for H; and 0; Transport through Porous Silicon (PS)Membranes”, K. R. Zavadil, Jun. 10, 1997].

Calculation of expected conductance values using a mean pore diameter of8.8 nm, a length of 50 microns and a pore size distribution given byHerino et al. [R. Herino, G. Bornchil, K. Baria, C. Bertrand and J. L.Ginoux, J. Electrochem. Soc. 134(8) 1987, 1994] produce equivalentvalues at 64% porosity. This degree of porosity is well within thelimits for the current density and HF concentration used foranodization. The response of these membranes has also been measured fora series of additional gases. The values from experiments were plottedas a function of the inverse square root of the gas molecular weight todemonstrate that this expected relationship is approximately adhered tofor transport in these pores. The line in this plot shows the expectedlinear dependence. The fact that significant deviation is observed atboth low and high molecular weights suggests that either excess flow issupported by an alternate transport mechanism (as previously observedfor H₂) or that much larger than expected variation in system pumpingspeed is occurring, skewing the measured values.

The conductance becomes difficult to measure at inlet pressures below a100 mTorr. This effect is visible in the data where the measuredconductance appears to decrease slightly at the lower pressures. Thiseffect possibly results from a drift in the baseline ion gauge readings,where the ion gauge is used to measure the pore outlet pressure. Thesystem was reconfigured to minimize this drift and extend the lowerlimit of quantification of conductance to inlet pressures of less than 1mTorr. The conductance measured for DMMP, measured at 2 mTorr,demonstrates how minimization of drift can yield conductance values atlow inlet pressures.

Defect structure appears to be the primary mechanism of gas transport inthese membranes, despite the correlation of experimental and expectedresults. From data for membrane response to elevated pore inletpressures in excess of the 10 Torr limit, it was found that beyond 10Torr the conductance starts to increase and eventually scales linearlywith pressure. This type of response is typical for a capillary in thetransition and eventually viscous flow regime. The response of an arrayof capillaries is calculated by using the integrated form of the Knudsenequation:C=nC _(M)[0.5 δ²+0.81 δ−0.009 ln(1+21 δ)]where n is the number of pores and δ is a unitless parameter equivalentto the ratio of the pressure diameter product at any value relative tothe value where the viscous conductance (C_(V)) is equivalent to themolecular conductance (C_(M)). Data show the expected variation inconductance with pressure if a weighted distribution of 5.3 to 11.3 nmpores were responsible for flow [R. Herino. G. Bornchil, K. Baria, C.Bertrand and J. L. Ginoux, J. Electrochem. Soc. 134(8) 1987, 1994]. Thefact that the transition flow regime is predicted to occur above a 1000Torr for this pore size distribution, indicates that a significantlylarger cross-sectional parameter structure must be responsible for theflow-observed in these membranes. A search for a reasonable fit to theexperimental data yields an average pore diameter of 16 microns for thetransmissive structure. Approximately 2 pores/membrane at a porediameter of 16 microns would be required to support the measured flowthrough these membranes.

An alternate explanation would be the presence of these defectsproducing the anomalous low transition pressure while the residual flowwould be supported by pores in the expected 5.3 to 11.3 nm range.

Results show a combined response (bi-modal distribution) of the expectedpore distribution at 10% porosity along with approximately 8,approximately 16 micron diameter pores distributed among the 5membranes. The resulting curve under-predicts the conductance throughthe transition flow region. Calculations show that attempts to alterthis bi-modal distribution by shifting the defect diameter towardapproximately 1 micron while increasing the contribution from theexpected pore population leads to a more severe underestimation ofconductance.

Results indicate that defect structure might be responsible for asignificant amount of flow in the membranes studied. The presence oflarge cross-section parameter defects could significantly impactapplications where pore side wall collisions control a desired process.Chemistry generated by surface catalysis or time dependent separationcreated by adsorption are two processes whose efficiency would bedrastically reduced due to a reduction in the number of side wallcollisions experienced by a molecule during pore transit. The previousexample of a bi-model pore size distribution provides some indication ofthe severity of this problem. In the above example, 84% of the flow isbeing carried by the large pores when pressure is kept belowapproximately 20 Torr. With viscous flow through these pores, thepercentage of flow increases to approximately 95% at 700 Torr. Thesevalues indicate that the intended, smaller diameter pores would play asecondary role in the desired process.

Further experimental results are disclosed below regarding fuel cells ofthe present invention. The aforementioned experiments show evidence ofmacro-structural defects in the porous silicon substrates as evidencedby low pressure thresholds for molecular-to-viscous flow in both p- andn-Si membranes. These defects were eliminated by drying the membranesusing super-critical CO₂ extraction. Previously, all side-supported Simembranes had been air dried after extended immersion in H₂O.

Based on these results, the structure responsible for the viscoustransport behavior was most probably inter-pore cracking resulting formthe capillary force present during drying. As a first attempt atminimizing capillary force, back-supported membranes were immersed inisopropanol—which has a surface tension approximately one-third that ofwater. Experimental results show that membranes can be dried from C₃H₇OHwithout detriment to the pore structure. However, a question remained asto whether the membranes could survive drying with H₂O penetrating thepore structure—an issue relevant for fuel cell applications wheremembranes are exposed to H₂O and may undergo some degree ofhydration/dehydration cycling.

Experiments wherein a hydration/dehydration cycle on a back-supportedmembrane (n-Si, approximately 0.005 to approximately 0.02 Ωcm) wereperformed. The membrane was taken from an immersion bath of C₃H₇OH andtransferred directly to de-ionized H₂O (18 M Ωcm) without drying. Thegoal was to utilize the miscibility of C₃H₇OH and H₂O to provide forfull wetting of the pores. The membrane remained immersed for a 16 hourperiod and was allowed to air dry. The results show that the membraneexhibits molecular flow for pressures up to 1000 Torr with apermeability for He of 7×10⁻⁸ mol s⁻¹m⁻²Pa⁻¹. This value lies within therange of He permeabilities measured for a combination of three otherback- and side-supported n-Si (approximately 0.005 to approximately 0.02Ωcm) membranes anodized under similar conditions (approximately 5×10⁻⁸to approximately 1×10⁻⁷ mol s⁻¹ m⁻² Pa⁻¹). The results support theconclusion that back-supported membranes survive mechanical stressesduring drying.

The low apparent porosity of these membranes was also examined. Acalculated expected permeability values of 5×10⁻⁶ mol s⁻¹ m⁻² Pa⁻¹ isbased on a mean pore diameter of 10 nm and a maximum porosity of 50%.The values measured suggest an apparent porosity of several tenths of apercent. Two explanations exist for these low values: tapered diameterpores (diameter decreasing from the anodization side inward) and a highdensity of terminated or branched pores that never penetrate the back ofthe membrane. Both of these possibilities would create a high volumetricporosity measured through mass loss but a low cross-sectional profilefor open pore area. The existence of a depletion layer and limited holemobility might be a possible explanation for pores not breaking through.The argument would be that the only pores that do break through arelocated close to the honeycomb support structure because of kineticlimitations of hole injection into and transport through the thinningmembrane overlayer. Variations in vertical porosity, presumably due tovarying pore diameter, have been observed by Herino et al. in 0.01 Ωcmn-Si previously (J. Electrochem. Soc., 134(8). 1987, p 1994).

To see if permeability might increase, the back of a side-supportedmembrane was etched. A membrane from a particular run was placed withthe support structure upward on a polyethylene support ring. A drop ofapproximately 49% by weight of hydrogen fluoride was placed on thesupport structure. The HF remained on the membrane back for 45 minuteswith no sign of penetration to the membrane front. The etching wasquenched in de-ionized H₂O and the membrane allowed to dry. Hepermeation testing produced a value of 5×10⁻⁶ mol s⁻¹ m⁻² Pa⁻¹. The risein this value at high pressure is the result of decreased system pumpingspeed. The increased gas transport results either from the exposure of 5times as many pores or a 1.7-fold increase in mean pore diameter.Removal of a depletion derived low porosity layer is plausible becausethe etch time used would result in removal of 9 nm of Si (assuming anetch rate of 0.2 nrn s⁻¹), a reasonable estimate of the depletion layerthickness expected in n-Si (S. R. Morrison, “Electrochemistry atSemiconductor and Oxidized Metal electrodes,” Plenum, 1980, p 69).Preferential pore widening at the back of the membrane is plausiblebecause we have indirect evidence that the etchant does not penetratethe full length of the pore and radial mass transport within the porewould limit the extent of pore side wall etching moving into themembrane.

Permeabilities add in reciprocal arguing that the removal of material ata restriction would have the greatest effect on transport. Thisexperiment did not distinguish between these two possibilities. The useof reactive ion etching, after anodization, would however help determinewhat the transport barrier is by studying the increase in permeabilitywith incremental removal of front and rear layers of the membrane. Fluidcondensation in the pores is not a possibility. Pore inlet gas pulsemeasurements for Xe and SF₆ show no sign of a time lag in themillisecond to second time regime. The difference in diffusioncoefficient for a free pore to a fluid-filled pore (10⁻² to 10⁻⁵ cm²s⁻¹) should produce a shift in lag time from the sub-millisecond regimeto a value of up to several seconds, depending on the extent of porefilling.

Results from the aforementioned experiments show that porous Simicromachined gas permeable membranes can be fabricated, that thetransport can be characterized as Knudsen flow, that permeability iscalculable for any gas, and that the back supported hexagonal supportallows for a robust membrane.

Pt Deposition

Initial deposition of platinum used electroless deposition of Pt insolutions between approximately 0.01 M and approximately 0.001 Mconcentration, at a pH of approximately 2.5 to approximately 4.5(adjusted through use of, for example, HCl). This deposition methodresulted in micron thick films. The goals for the Pt catalyst layer arethat the Pt layer must not obscure pores and block flow of gas. Othercatalytic materials may be used.

Fuel Cell Performance

Using Nation films, fuel cells were tested in a sandwich configuration.Performance is shown in FIG. 17 as a polarization curve.

Permeation measurements were made using equipment to show that hydrogenflows through the anode and cathode of an apparatus of the presentinvention at approximately 5 ml min⁻¹. However, blocking the cathodefrom air flow did not completely shut down the fuel cell. Performancedropped by approximately 50% suggesting that oxygen was still able todiffuse into the perimeter of the fuel cell.

In general, efficiency of hydrogen utilization results from, forexample: improvements in hydrogen flow; improvements in the porosity ofthe gas diffusion support structure, including the catalyst (e.g.,platinum) coating; improvements in electrolyte deposition; andimprovements in the catalyst/electrolyte interface. Electrolytedeposition is improved, for example, by depositing membranes fromsolution and/or hot pressing of polymer films, e.g., NAFION® films.

Referring to FIG. 16, a porous silicon-based membrane of the presentinvention is shown. The micrograph is of a n⁺-Si unsupportedapproximately 50 mm membrane anodized at approximately 50 mA cm⁻².

Fuel cells of the present invention are suitable for use in a wide rangeof applications as a suitable replacement for batteries. Applicationsrange from consumer electronic devices (cell phones, PDAs, laptopcomputers). Applications such as integrated or removable chip-basedpower sources for micromachinines is particularly compelling. In thisapplication, the fuel cell power converter can be fabricated on the samewafer as the micromachine to enable chip based integrated microsystemsthat have the functions of, for example, sensing, thinking,communicating, communicative functions, and micromachine actions.Microrobotics, for example, enable by such fuel cell on a chip power issuitable for, but not limited to, robotic vehicles.

As mentioned above, the present invention comprises the ability forconfiguration and operation at high temperatures. High temperature(approximately 100° C. to greater than approximately 1000° C.)configuration/operation of apparatus of the present invention simplifiesfueling and enables direct utilization of hydrocarbon or alcohol fuels.High temperature operation/configuration also enables internal reformingof these fuels to produce hydrogen as the primary fuel.

According to the present invention, a high temperatureoperation/configuration is enabled through the use of known oxideconductors such as zirconia based electrolytes (e.g., yttria stabilizedzirconia or YSZ) on platinum or other refractory catalysts and currentcollectors on top of the silicon membrane, preferably comprised ofsilicon nitride. In this simple configuration, platinum or othersuitably refractory materials perform as both the dispersed catalyst andas the current collector.

Possible YSZ electrolyte compositions that could be deposited bysolution methods, sputter methods, slurry, tape or other methods includethe formulation 0.9 ZrO2+0.1 Y2O3 as well as the formulation 0.9ZrO2+0.04 Y2O3+0.06 Sc2O3.

According to the present invention, both oxide conducting and protonconducting high temperature electrolytes can be used. High temperaturecatalyst materials that are optionally deposited by solution, sputter,CVD or other methods include Noble metals: Pt, Au, Ag, Pd, Ag/Pd alloy,as well as non-Noble metals: Ni, Co, Cu, Fe and oxides: PrO₂, CeO₂,In₂O₃ as well as oxide compositions such as manganites and cobaltites.

The present invention also allows for rapid device development andoptimization. The example shown above wherein the fabrication approachyielded 18 fuel cell devices (many thousand of individual cells) on asingle wafer enables the combinatorial optimization of componentmaterials (e.g., catalysts, membranes and device designs (electrodegeometries, membrane pore size, water management techniques, . . . ). Inthis manner individual device geometries and other design parameters areoptionally varied systematically through mask design, catalysts andmembranes could be systematically varied through controlled deposition(masks for CVD or sputter deposition) or controlled delivery of liquidformulations (e.g., ink jet depositions).

Parallel diagnostics of the multi-device wafer is accomplished usingembedded sensors, optical screening or thermal imaging of themulti-device wafer for thermal measurement of individual devices orthrough “docking” a wafer characterization module to the wafer toprovide parallel measurement of individual device performance (gasconsumption, voltage, current, heat, pH).

In this manner many device configurations can be evaluated in a parallelrather than traditional cut and try serial development fashion.Individual device designs or formulations can be rapidly optimized andknowledge of correlation of device design/material trade-offs assessed.For example, 10⁴ fuel cell electrode pairs are fabricated on a chip. Inthese chips, for example, the catalyst formulation is varied in a knownway across the surface area of the wafer. Next, each fuel cell isexamined, e.g. measurement of the polarization curve for 10 seconds.Overall, this combinatorial technique compresses millions ofperson-hours of experimental work into approximately one day.

Various embodiments of the present invention allows for, for example,fuel cell integration with electronic devices; mass production of fuelcells using automated tools of Semiconductor Manufacturing industry;thinner membrane and consequently higher proton mobility; semiconductortype interconnects and thus higher current collection efficiency; aflexible prismatic form factor; scalability; elimination, in someembodiments, of endplates, bolts, or tie rods; higher energy density;rapid optimization; integration with a chemical or metal hydride H₂storage system; chip-based electrolyzers for recharging; powermanagement integration; liquid fuel integration (e.g. MeOH); flexibleuse of pre-cast or other membranes; control over a 3-way interfacemicrostructure by building an interface from the “ground up” from apolished, rigid Si wafer surface to catalyzed pores with an engineeredpore size; placement of catalyst and ionomer at a controlled depth intothe pore by electrochemical deposition and spin casting; and reductionin ionomer membrane thickness.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

Although the invention has been described in detail with particularreference to these preferred embodiments, other embodiments can achievethe same results. Variations and modifications of the present inventionwill be obvious to those skilled in the art and it is intended to coverin the appended claims all such modifications and equivalents. Theentire disclosures of all references, applications, patents, andpublications cited above are hereby incorporated by reference.

1. A fuel cell comprising: a dielectric substrate material having upperand lower surfaces; a porous film disposed on said upper surface of saiddielectric substrate material and comprising a silicon-based thin filmmembrane; said porous film comprising at least one electrode; andchannels extending through said dielectric material from said uppersurface to said lower surface.
 2. The fuel cell of claim 1 additionallycomprising a fuel source disposed in relation to apertures of channelson said lower surface of said dielectric material.
 3. The fuel cell ofclaim 2 wherein said fuel source comprises at least one member selectedfrom the group consisting of hydrogen gas, alcohols, and other compoundscontaining redox pairs.
 4. The fuel cell of claim 3 wherein an oxygensource of said fuel cell is ambient air.
 5. The fuel cell of claim 1wherein said porous film is etch-processed.
 6. The fuel cell of claim 1wherein said porous film comprises a solid electrolyte.
 7. The fuel cellof claim 6 wherein said solid electrolyte comprises a proton exchangepolymer.
 8. The fuel cell of claim 7 wherein said proton exchangepolymer comprises a perfluorosulfonate ionomer.
 9. The fuel cell ofclaim 8 additionally comprising a moisture cap.
 10. The fuel cell ofclaim 7 wherein said solid electrolyte is disposed on said dielectricsubstrate by a method comprising spin coating, lamination, or spraying.11. The fuel cell of claim 6 wherein said solid electrolyte comprises anoxide conducting electrolyte.
 12. The fuel cell of claim 11 wherein saidoxide conducting electrolyte comprises a zirconia-based electrolyte. 13.The fuel cell of claim 11 wherein a catalyst is disposed on said solidelectrolyte and wherein said catalyst comprises a refractory material.14. The fuel cell of claim 13 wherein said refractory material isselected from the group consisting of platinum, Ru, or a Pt—Ru alloy.15. The fuel cell of claim 11 having operation temperatures betweenapproximately 100° C. and approximately 1000° C.
 16. The fuel cell ofclaim 7 wherein said solid electrolyte comprises a high temperatureproton-conducting electrolyte.
 17. The fuel cell of claim 1 wherein saidthin film membrane comprises at least one member selected from the groupcomprising silicon nitride and silicon carbide.
 18. The fuel cell ofclaim 1 wherein said silicon-based thin film membrane comprises a lowstress, pre-tensioned membrane.
 19. The fuel cell of claim 1 whereinsaid silicon-based thin film membrane comprises a thickness betweenapproximately 0.5 μm and approximately 20 μm.
 20. The fuel cell of claim19 wherein said silicon-based thin film membrane comprises a thicknessbetween approximately 1 μm and approximately 10 μm.
 21. The fuel cell ofclaim 20 wherein said silicon-based thin film membrane comprises athickness between approximately 1 μm and approximately 5 μm.
 22. Thefuel cell of claim 21 wherein said silicon-based thin film membranecomprises a thickness of approximately 1 μm.
 23. The fuel cell of claim1 wherein said silicon-based thin film additionally comprises apatterned and etched geometric array comprising an approximately 1 μmdiameter and an approximately 2 μm pitch.
 24. The fuel cell of claim 23wherein said silicon-based thin film membrane comprises a thickness ofapproximately 1 μm.
 25. The fuel cell of claim 1 wherein saidsilicon-based thin film membrane is formed through low pressure chemicalvapor deposition.
 26. The fuel cell of claim 1 wherein pores are formedin said silicon-based thin film membrane through reactive ion etching.27. The fuel cell of claim 1 wherein said thin film comprises a mask foran anodization etching process.
 28. The fuel cell of claim 1 whereinsaid porous film comprises a conductive layer.
 29. The fuel cell ofclaim 28 wherein said conductive layer comprises at least one materialselected from the group consisting of gold, aluminum, platinum, othermetals, metal alloys, and a conductive organic material.
 30. The fuelcell of claim 28 wherein said conductive layer comprises at least onecatalyst.
 31. The fuel cell of claim 28 wherein said catalyst isdisposed on said conductive layer to comprise a porous catalyst.
 32. Thefuel cell of claim 30 wherein said catalyst comprises a materialselected from the group comprising platinum, ruthenium, andplatinum-ruthenium alloys.
 33. The fuel cell of claim 30 wherein saidcatalyst comprises a catalyst for high temperature operation comprisedfrom a member of the group consisting of alloys of Noble metals,non-Noble metals, metal oxides, and oxide compositions.
 34. The fuelcell of claim 33 wherein said Noble metals are selected from the groupcomprising Pt, Au, Ag, Pd, and Ag/Pd alloys.
 35. The fuel cell of claim33 wherein said non-Noble metals are selected from the group comprisingNi, Co, Cu, and Fe.
 36. The fuel cell of claim 33 wherein said metaloxides are selected from the group comprising PrO₂, CeO₂, and InO₃. 37.The fuel cell of claim 33 wherein said oxide compositions are selectedfrom the group comprising manganites and cobaltites.
 38. The fuel cellof claim 30 wherein said conductive layer and said catalyst are disposedby at least one method selected from the group consisting of chemicalvapor deposition, physical deposition, evaporation, and ink deposition.39. The fuel cell of claim 1 wherein said at least one electrodecomprises at least one anode and at least one cathode.
 40. The fuel cellof claim 39 wherein said anode and said cathode comprise differentsurface areas.
 41. The fuel cell of claim 40 wherein said surface areaof said anode comprises between approximately two times andapproximately ten times less surface area than said surface area of saidcathode.
 42. The fuel cell of claim 41 wherein said surface area of saidanode comprises approximately four times less surface area than saidsurface area of said cathode.
 43. The fuel cell of claim 42 wherein saidsurface area of said anode comprises a width of approximately 40 μm anda length of approximately 1 cm, and wherein said cathode comprises awidth of approximately 160 μm and a length of approximately 1 cm. 44.The fuel cell of claim 39 wherein said at least one anode comprises awidth of between approximately 10 μm and approximately 200 μm.
 45. Thefuel cell of claim 39 wherein said at least one cathode comprises awidth of between approximately 10 μm and approximately 200 μm.
 46. Thefuel cell of claim 37 wherein said anodes and said cathodes areinterposed in interdigitated planar relation.
 47. The fuel cell of claim46 wherein said anodes and said cathodes comprise a configurationselected from the group consisting of parallel, series, or combinedparallel-series configurations.
 48. The fuel cell of claim 39 whereinsaid at least one anode and said at least one cathode compriseserpentine or spiral patterns.
 49. The fuel cell of claim 1 wherein saiddielectric substrate comprises a silicon-based material.
 50. The fuelcell of claim 49 wherein said dielectric substrate comprises siliconnitride.
 51. The fuel cell of claim 1 wherein said channels are formedby joining at least two micromachined wafers.
 52. The fuel cell of claim1 wherein said channels comprise pores within said dielectric substrate.53. The fuel cell of claim 52 wherein said dielectric materialsurrounding said channels comprises a dielectric barrier.
 54. The fuelcell of claim 53 wherein every other said dielectric barrier between ananode and a cathode comprises a conductive layer coating.
 55. The fuelcell of claim 53 wherein said dielectric barrier comprises a widthbetween approximately 10 μm and approximately 50 μm.
 56. The fuel cellof claim 55 wherein said dielectric barrier comprises a width ofapproximately 25 μm.
 57. The fuel cell of claim 52 wherein said poresare formed by reactive ion etching.
 58. The fuel cell of claim 52wherein said pores comprise at least one flow path for providing fuel tosaid at least one electrode.
 59. The fuel cell of claim 1 wherein anaperture of said channels corresponds approximately to surface areas ofsaid electrode.
 60. The fuel cell of claim 1 wherein pores of saidporous film have a diameter of between approximately 5 nm andapproximately 1000 nm.
 61. The fuel cell of claim 1 wherein surfaceswithin said cell comprise geometries selected from the group comprisingplanes, curved surfaces, flexible surfaces, and cylinders.
 62. The fuelcell of claim 61 wherein apertures of said cylinders may comprisegeometric figures selected from the group consisting of triangles,rectangles, circles, polygons, and ellipses.
 63. A bipolar fuel cellcomprised of two fuel cell units as in claim 1 wherein said uppersurfaces of said units are in joined connected relation.
 64. The bipolarfuel cell of claim 63 wherein said at least one electrode of one of saidunits comprises an anode and said at least one electrode of remainingsaid unit comprises a cathode.
 65. The fuel cell of claim 1 wherein saidlower surface of said dielectric substrate material comprises a coatingcomprising an ohmic contact.
 66. The fuel cell of claim 65 wherein saidohmic contact is comprised of a material selected from the groupcomprising aluminum, gold, silver, other metals, and metal alloys. 67.The fuel cell of claim 1 additionally comprising micro-switchingdevices.
 68. The fuel cell of claim 67 wherein said micro-switchingdevices selectively interconnect said electrodes.
 69. The fuel cell ofclaim 67 additionally comprising micro-switching devices within saidchannels for controlling a fuel flow.
 70. The fuel cell of claim 1additionally comprising cooling means for reducing a fuel celltemperature.
 71. The fuel cell of claim 1 formed entirely bysemiconductor manufacturing methods.
 72. A fuel cell comprising an etchand anodization processed, porous electrode.
 73. The fuel cell of claim72 wherein said electrode is silicon-based and wherein said silicon isdoped.
 74. A fuel cell comprising: a dielectric substrate materialhaving upper and lower surfaces; a porous film disposed on said uppersurface of said dielectric substrate material and comprising a solidelectrolyte comprising a proton exchange polymer comprising aperfluorosulfonate ionomer; said porous film comprising at least oneelectrode; channels extending through said dielectric material from saidupper surface to said lower surface; and a moisture cap.
 75. A fuel cellcomprising: a dielectric substrate material having upper and lowersurfaces; a porous film disposed on said upper surface of saiddielectric substrate material; said porous film comprising at least oneelectrode comprising at least one anode and one cathode comprisingdifferent surface areas; and channels extending through said dielectricmaterial from said upper surface to said lower surface.
 76. A fuel cellcomprising: a dielectric substrate material having upper and lowersurfaces; a porous film disposed on said upper surface of saiddielectric substrate material; said porous film comprising at least oneelectrode comprising at least one anode and one cathode, wherein saidanodes and said cathodes are interposed in interdigitated planarrelation; and channels extending through said dielectric material fromsaid upper surface to said lower surface.
 77. A fuel cell comprising: adielectric substrate material having upper and lower surfaces; a porousfilm disposed on said upper surface of said dielectric substratematerial; said porous film comprising at least one electrode comprisingat least one anode and one cathode, wherein said at least one anode andsaid at least one cathode comprise serpentine or spiral patterns; andchannels extending through said dielectric material from said uppersurface to said lower surface.
 78. A fuel cell comprising: a dielectricsubstrate material having upper and lower surfaces and comprisingsilicon nitride; a porous film disposed on said upper surface of saiddielectric substrate material; said porous film comprising at least oneelectrode; and channels extending through said dielectric material fromsaid upper surface to said lower surface.
 79. A fuel cell comprising: adielectric substrate material having upper and lower surfaces; a porousfilm disposed on said upper surface of said dielectric substratematerial; said porous film comprising at least one electrode; andchannels extending through said dielectric material from said uppersurface to said lower surface, wherein said channels comprise poreswithin said dielectric substrate, wherein said dielectric materialsurrounding said channels comprises a dielectric barrier, and whereinevery other said dielectric barrier between an anode and a cathodecomprises a conductive layer coating.
 80. A fuel cell comprising: adielectric substrate material having upper and lower surfaces; a porousfilm disposed on said upper surface of said dielectric substratematerial; said porous film comprising at least one electrode; andchannels extending through said dielectric material from said uppersurface to said lower surface, wherein said lower surface of saiddielectric substrate material comprises a coating comprising an ohmiccontact.
 81. A fuel cell comprising: a dielectric substrate materialhaving upper and lower surfaces; a porous film disposed on said uppersurface of said dielectric substrate material; said porous filmcomprising at least one electrode; channels extending through saiddielectric material from said upper surface to said lower surface; andmicro-switching devices.
 82. A fuel cell comprising: a dielectricsubstrate material having upper and lower surfaces; a porous filmdisposed on said upper surface of said dielectric substrate material;said porous film comprising at least one electrode; and channelsextending through said dielectric material from said upper surface tosaid lower surface; and wherein one or more widths are provided from thegroup consisting of at least one said anode comprising a width ofbetween approximately 10 μm and approximately 200 μm, at least one saidcathode comprising a width of between approximately 10 μm andapproximately 200 μm, and said dielectric barrier comprising a widthbetween approximately 10 μm and approximately 50 μm.